Cellulose, in particular chemical pulp, which is to be acetylated, must be activated before the acetylation. Normally the activation occurs because the pulp is treated with acetic acid, (see Ullmann's Encyclopedia of Industrial Chemistry, Fifth Complete Revised Edition, Volume A 5, page 440--VCH Verlagsgesellschaft mbH, D-6940 Weinheim, Federal Republic of Germany 1986).
Depending on process conditions, with this known activation process it may happen that homogeneous activation of the pulp does not occur, because the crystalline parts of the pulp are not readily accessible to the pure acetic acid that is glacial acetic acid which is used. The result is that in the following acetylation step, the desired cellulose triacetate stage is reached in the amorphous zones of the pulp prior to the crystalline parts.
From the journal "Acta Polymerica" 32 (1981), No. 3, pages 164 to 171, the use of liquid ammonia for the activation of cellulose is known, namely at normal pressure and at a temperature of 50.degree. to 60.degree. C. The use of liquid ammonia for the activation of cellulose offers the advantage over glacial acetic acid that also the crystalline parts of the cellulose become more accessible due to the expansion of the crystal lattice. However, because of the low temperature, the practical application of this process is rather costly, which may explain the fact that this process is not being employed industrially.
Further, the use of liquid ammonia for the activation of cellulose contained in feed products under application of elevated pressure is known from European Patent Application 0 077 287. According to the process described in this publication, the cellulose-containing material is treated at high pressure with liquid ammonia, and the pressure is rapidly reduced to atmospheric pressure, so that the ammonia boils and the cellulose is defibrated.
The process according to the European Patent Application 0 077 287 is not suitable for the activation of pulp that is subsequently to be acetylated because the ammonia still remaining in the pulp after the expansion will react with the acetylation agent acetic acid and with the acetylation catalyst sulfuric acid, with formation of salts. Due to this fact, the ammonia, in bound form, remains during the acetylation process and in the subsequent saponification process, and at the end, after the precipitation of the cellulose acetate, it goes into the waste water, which is highly undesirable.
When cellulose is activated with liquid ammonia, washing the ammonia out with water at the end of the process is indeed known (see the journal Faserforschung und Textiltechnik 25 (1974), No. 2, pages 57 to 60--Zeitschrift fur Polymerforschung), but according to the above mentioned literature source, the activation with the liquid ammonia occurs at low temperatures or under cooling, and this fact described in "Acta Polymerica" 32 (1981), No. 3, pages 164 has been mentioned in connection with the activation process to 171. This makes the practical application of such an activation process rather costly.